1. Field of the Invention
This invention relates to position sensors for supplying data indicative of the position of a movable member, and is more particularly concerned with a detection circuit for a position sensor having two impedance elements, at least one of which has an impedance which varies in dependence on the position of the movable member.
2. Description of the Related Art
It is known to provide a variable impedance position sensor comprising a variable inductor and a reference inductor connected in series or a differential pair of variable inductors. The detection circuitry of such a sensor commonly incorporates a resonating oscillator which is driven at a controlled voltage level, and a detector for detecting the voltage at the node between the two inductors and synchronously demodulating this voltage to provide an output signal indicative of the position of the movable member.
Most sensors of this type operate at low frequencies in order that the drive voltage and the synchronous demodulation can be controlled easily. However, the use of low frequencies requires that high inductance values are used for the inductors, and this usually requires a large number of turns of small diameter wire in the inductors which can render the sensor both fragile and expensive.
Alternatively sensors of this type are provided which have low inductance values and are designed to be operated at high frequencies. Such sensors can be of simplified and more robust construction because the inductors have fewer turns and can be produced using printed circuit board layout techniques or very much simplified wound coils. However the high frequency operation of such sensors renders the synchronous demodulation of the detected voltage from the oscillator more difficult and subject to some inconsistencies with variation in the position of the movable member. In this regard it is important that the amplitude detection of the oscillator output occurs as closely as possible to the zero-crossing point of the current in the resonant circuit in order to render any variations in the loss resistance of the impedance negligible.
U.S. Pat. No. 4,644,570 discloses a sensor of this type which provides two signals having periods determined by the magnitude of the two inductors, and logic circuitry for providing an output signal indicative of the difference in duration between these two periods divided by the sum of the periods, a predetermined gain being applied to the output signal to provide a signal indicative of the position of the movable member. However such detection circuitry is complex and may also require some temperature compensation.
Reference is also made to other sensors of this type in EP 0215840B, GB 2053487, U.S. Pat. No. 4,450,443, U.S. Pat. No. 4,253,079, U.S. Pat. No. 4,425,511, U.S. Pat. No. 4,507,638, U.S. Pat. No. 4,982,156 and JP 57013303A.